When a magnetic field is applied to a conductor carrying a current, in a direction at right angles to the current, an electromotive force is produced across the conductor in a direction perpendicular to both the current and to the magnetic field. This effect, known as the Hall effect after E. H. Hall who discovered it in thin metallic foils in 1879, has become one of the most powerful tools for studying the electronic properties of semiconductors. As it is most commonly used today, the measurement of the Hall voltage enables a process engineer to determine the number of current carriers per unit volume within a semiconductor device, and also whether they are positively or negatively charged.
In the past, certain classes of semiconductor devices have utilized the Hall effect for for particularized applications. For example, U.S. Pat. No. 4,516,144 discloses a magnetically sensitive semiconductor device used to sense crankshaft angle positions in automotive system. In the operation of that device, carriers from an emitter region travel through a base region toward one or the other of a pair of spaced-apart collector regions. The carriers are deflected toward one or the other collector regions according to the polarity of a perpendicularly applied magnetic field. The strength and direction of the magnetic field is determined by the crankshaft angle position.
The presence of a magnetic field also works to change the properties of light travelling through it. A device which changes the irradiance (or direction) of the light passing through it is called a modulator. There are several general types of modulators; namely, mechanical, electro-optic, magneto-optic, elasto-optic, and passive modulators. Optical modulators are described generally in U.S. Pat. Nos. 4,079,429 and 3,988,704.
Many modulators rely on well-understood optical effects and principles such as the Kerr effect, the Faraday effect, the acousto-optic effect, etc. For example, liquid Kerr cells filled with nitrobenzene and placed between crossed polarizers have been used extensively as optical switches or modulators in place of ordinary Pockels cells. Modulation at frequencies up to 10.sup.10 Hz have been obtained.
The Faraday effect is a magneto-optic effect of real interest for optical modulators. In the Faraday effect, a beam of plane polarized light passing through a substance subjected to a magnetic field is observed to rotate by an amount proportional to the magnetic field component parallel to the direction of propagation. Important in the Faraday effect, is the fact that the rotation of the plane of polarization is independent of the direction of propagation.
For some time, researchers have attempted to apply the above described effects for the purpose of developing a large capacity computer memory. It is believed that optical effects could overcome the problems and constraints inherently associated with magnetic or conventional semiconductor memories. Such an optical memory would be capable of storing large amounts of information in a relatively small area and operate at extremely high data rates. Because of its great potential, it is particularly desirable to develop a semiconductor magnetic memory cell capable of acting as an optical modulator. However, the integration of a magnetic memory storage element and a semiconductor sensor to produce a device which is effective as an optical modulator has thus far proven to be a formidable task. Examples of optical modulators which utilize semiconductor materials are found in U.S. Pat. Nos. 4,727,341 and 4,837,526.
As will be seen, the present invention integrates a magnetic storage element with a solid-state sensor and/or amplifier to form a novel memory cell effective as a spatial light modulator. Data is stored in the form of magnetized patches or domains in a magnetic material placed in close proximity to a semiconductor sensor. In a preferred embodiment, the magnetic field is directed vertically through the semiconductor sensor to generate a transverse voltage in accordance with the Hall effect. The Faraday or Kerr effects are utilized to rotate the plane of polarization of laser light incident upon the memory cell for read out. The direction of the change in the polarization of the laser beam on passing through or being reflected from the memory elements depends on the directions of magnetization. Magnetizations of the memory elements in one direction may represent `ones`; in the opposite direction `zeros`. The spatial optical modulator of the present invention is such that it lends itself to numerous embodiments and a variety of applications.